Why does a grassland grow much more in some years than others, even when annual weather averages do not explain it?

Details

Background. Annual above-ground net primary productivity (ANPP) in mesic grasslands—grasslands with moderate moisture—is highly variable over time. Annual precipitation or average annual temperature can explain some of this variability, but much remains unexplained.

The researchers examined 25 years of productivity data from a mesic grassland at Konza Prairie in north-eastern Kansas. They investigated precipitation and temperature during periods relevant to the seasonal development and growth cycle of the dominant C4 grasses, along with these grasses’ production of flowering stalks. Using structural equation modelling (SEM), they assessed both direct effects of climate during these biologically relevant periods on ANPP and indirect effects through flowering.

ANPP showed strong positive relationships with flowering stalk production by the dominant C4 grasses, precipitation during their vegetative growth period (15 April–14 July), precipitation during their flowering stalk elongation period (15 July–14 August), and fire. High temperatures during flowering stalk elongation were linked to lower flowering stalk production, producing a negative indirect effect on ANPP in the modelling. There was little evidence that the previous year’s total annual precipitation or mean annual temperature affected ANPP.

Including flowering stalk production and separating climate variables into periods relevant to the grasses’ seasonal development increased the proportion of observed ANPP variation explained by six models, covering different topographic positions and burn regimes, from an average of 22% to 48%. The best model explained 61% of ANPP variation.

Synthesis. The links among climate during periods relevant to the dominant C4 grasses’ seasonal development and growth, their flowering stalk production, and ANPP improve the ability to predict productivity in mesic grasslands, an ecologically and economically important ecosystem.